This invention relates generally to a turbofan engine control system and, more particularly, to a method and apparatus for scheduling the opening of variable bleed doors to maintain a proper interflow between a booster stage and a downstream compressor.
It is common in the manufacture of turbofan engines to increase the overall engine compression ratio by adding one or more booster stages upstream of the compressor. These booster stages are normally driven independently from the compressor and are disposed within the core engine compressor flow passage. Accordingly, in order to maintain the proper relationship between the amount of air being discharged from the booster and that being pumped through the compressor over variable modes of operation, it has been found advantageous to provide a bypass valve mechanism such as that shown in U.S. Pat. No. 3,638,428, issued to Gerald E. Shipley, et al on Feb. 1, 1972 and assigned to the assignee of the present invention, for bypassing a portion of the air pressurized by such booster stages during off-design engine operation so as to prevent aerodynamic stall caused by the back pressuring of the booster stages because of the inability of the core compressor to accept all of the air supplied.
A common approach is to schedule the opening of the variable area bypass doors as a function of core speed only, and for optimization either at a particular engine operating point or over a relatively narrow band of engine operation, such as, for example, a particular flight Mach number at high altitude cruise. However, since there is a different core-booster rotor speed-match for different flight Mach numbers, then, at operating points which are higher or lower than that of the design, there may be either a loss of performance by the bleeding off of too much air or the loss of stall margin by the failure to bleed off enough air.
The mismatch between booster and core is made more complicated by the difference in the moment of inertia of the two rotor systems. The core rotor with the smaller moment of inertia will accelerate and decelerate faster than the booster or fan rotor, so that during periods of acceleration, the bleed doors may be completely closed, but during periods of deceleration, the bleed door opening requirements for meeting the desired stall margin may very well be greater than that corresponding to the designed engine operating point. If the resultant loss in booster stall margin is caused by the back pressuring on the booster, then aerodynamic stall of the booster stages could result.
A further condition which may tend to intensify the problem of core-booster mismatch, is that of variation in the condition of the core. It is recognized that the quality of a core varies due to production tolerances or engine in-service deterioration, and a system which schedules the opening of the bleed doors solely as a function of core speed will tend to have this mismatch intensified by a degradation of the core quality.
It is therefore an object of this invention to provide a bypass modulating system which allows for the maintenance of a desired booster stall margin regardless of the flight or power level condition.
Another object of this invention is the provision in a bleed control system for maintaining a desired booster stall margin during periods of engine deceleration.
Yet another object of this invention is the provision for a booster bleed control system which does not degrade the booster stall margin as a result of new engine manufacturing variations or in-service quality deterioration.
Still another object of this invention is the provision for a booster bleed modulation system which automatically compensates for engine rpm transients so as to prevent a loss in stall margin.
A further object of this invention is to provide a bleed door control system which is simple in construction and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.
Briefly, in accordance with one aspect of the present invention, both the speed of the core and that of the booster are sensed and the opening of the bleed doors is modulated in response thereto over all ranges of engine operation. In this way, the difference between the core speed, which is indicative of corrected core flow, and the booster speed, which is indicative of booster corrected flow, provides an indication of the amount of air that is to be bled off.
In accordance with another aspect of the invention, the steady-state bleed schedule, which schedules the opening of the bleed door solely as a function of the core speed is modified in response to the speed of the booster. The schedule may be accordingly increased or decreased so as to increase the booster stall margin or to increase the engine performance, respectively.
By yet another aspect of this invention a fan speed reference is generated by comparing the actual core speed with a steady-state reference schedule, and then an actual fan speed is compared with the referenced fan speed to obtain a biasing factor which in turn is applied to the basic steady-state bleed door schedule so as to increase or decrease the amount of air that is bled off.
The present invention provides for the maintenance of a desired booster stall margin regardless of the power level operating condition, and allows trade-offs between the stall margin and steady-state performance at any or all power level conditions. Since it takes into consideration both the amount of air which the core is accepting and the amount of air which the booster is delivering, engine rpm transients are automatically compensated for so as to prevent a resulting loss in stall margin. Further, the booster stall margin is made insensitive to new engine manufacturing variations and to in-service quality deterioration.
In the drawings as hereinafter described, the preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.